Adhesives have been used for a variety of marking, holding, protecting, sealing and masking purposes. Adhesive tapes generally comprise a backing, or substrate, and an adhesive. One type of adhesive which is particularly preferred for many applications is represented by pressure sensitive adhesives. Pressure sensitive adhesives (PSAs) are well known to one of ordinary skill in the art to possess certain properties including the following: (1) aggressive and permanent tack, (2) adherence with no more than finger pressure, (3) sufficient ability to hold onto an adhered, and (4) sufficient cohesive strength. As applications for pressure-sensitive adhesives have increased substantially in recent years, performance requirements have become increasingly demanding.
Materials that have been found to function well as pressure sensitive adhesives are polymers designed and formulated to exhibit the requisite viscoelastic properties resulting in a desired balance of tack, peel adhesion, and shear strength. The most commonly used polymers for preparation of pressure sensitive adhesives are various (meth)acrylate based copolymers, natural rubber, synthetic rubbers, and silicones.
The pressure sensitive adhesive materials known from the prior art do not often provide sufficient tack to various types of substrate, including the so-called LSE and MSE substrates, i.e. substrates having respectively a low surface energy and a medium surface energy. In particular, the peel force or shear resistance on these challenging-to-bond substrates, particularly the LSE and MSE substrates, do not often fulfill the requirements, especially under environmental stress like altering temperatures and humidity.
This deficiency may partly be overcome by the addition of higher amounts of tackifiers. However, the excessive use of tackifiers may often detrimentally affect critical characteristics of pressure sensitive adhesives such as e.g. shear resistance and cohesive strength, and may raise the glass transition temperature of the corresponding adhesive. Furthermore, tackifiers may migrate into the substrate to which the adhesive tape is bonded and may lead to an undesired color change or stability reduction.
Without contesting the technical advantages associated with the pressure sensitive adhesive films known in the art, there is still a need for a stable and cost-effective pressure sensitive adhesive assembly having improved and versatile adhesion characteristics, in particular with respect to peel forces and shear resistance on various types of difficult to adhere surfaces, such as in particular LSE and MSE substrates.
Other advantages of the multilayer pressure sensitive adhesive (PSA) assembly and methods of the disclosure will be apparent from the following description.